Recently, facsimile equipment, printers, etc., have become remarkably developed, and a heat-sensitive recording system comprising a combination of, for example, a heat-sensitive recording paper having a heat-sensitive color forming layer containing a colorless dye such as crystal violet lactone and a phenol compound and a thermal head as described, for instance, in Japanese Patent Publication No. 14039/70 (corresponding to British Patent Publication No. 1,135,540 A), etc., is widely employed for such apparatus.
The heat-sensitive recording system has many advantages that the recording paper is of primary coloring, the system does not require liquid development, the recording apparatus can be simplified, the costs for recording papers, recording apparatus, etc., are low, recording can be performed in a non-impact manner without generating noise, etc., and hence this system has gained a steadfast position as a low-speed recording system. However, a significant disadvantage of the heat-sensitive recording system is that the recording speed is lower than those of other recording systems such as electrostatic recording, and hence the recording system has not yet been employed in high-speed recording.
The main reason that the aforesaid heat-sensitive recording system has not been applicable for high-speed recording in heat-sensitive recording is that the heat conduction between a thermal head and a heat-sensitive recording paper which is brought into contact with the thermal head is insufficient, whereby a sufficient recording density is not obtained.
A thermal head composed of an assembly of dot-form electron resistance heating elements generates heat by recording signals to melt and color a heat-sensitive color forming layer in contact with the thermal head. In this system, for obtaining clear and high-density recording, it is required that the dot reproducibility is good, that is, it is required that a thermal head is brought into contact with a heat-sensitive color forming layer as closely as possible in order to efficiently conduct heat transfer and form completely colored dots corresponding to the form of the dot heating elements of the thermal head at high-speed. However, at present, only a few percent of the amount of heat generated at the thermal head is transferred to the heat-sensitive color forming layer, and hence the heat transfer efficiency is very low.
Various methods for improving the smoothness of a heat-sensitive color forming layer for bringing a heat-sensitive color forming layer into contact with a thermal head as closely as possible have been proposed.
For example, Japanese Patent Publication No. 20142/77 describes that the surface of a heat-sensitive color forming layer is treated to provide a surface smoothness of 200 to 1,000 sec. in Beck smoothness. Japanese Patent Application (OPI) No. 115255/79 (The term "OPI" as herein used refers to a "published unexamined Japanese patent application".) describes that the heat-sensitive color forming layer having a surface smoothness of from 200 to 1,000 sec. in Beck smoothness can respond to a heat pulse of as short as about 5 or 6 milli-seconds and for performing high-speed recording of shorter than 1 milli-second, it is necessary that the surface of a heat-sensitive color forming layer is treated to provide a surface smoothness of higher than 1,100 sec. in Beck smoothness. However, if the surface of a heat-sensitive coloring layer is smoothened to higher than 1,100 sec. in Beck smoothness, colored fog is formed due to pressure. The formation of colored fog is prevented by previously improving the smoothness of the surface of a base paper for a heat-sensitive recording paper to higher than 500 sec. in Beck smoothness. Furthermore, Japanese Patent Application (OPI) No. 156086/78 describes that the surface roughness Ra of the surface of a heat-sensitive color forming layer is reduced to lower than 1.2 .mu.m, and the glossiness thereof is reduced to lower than 25% of its original value.
In the above-described conventional techniques for improving the smoothness of heat-sensitive color forming layers of heat-sensitive recording papers, the smoothness of the heat-sensitive color forming layers is improved by a calender treatment simply using a super calender, a machine calender, a gloss calender, etc. The calender treatment is applied to a base paper only or to a base paper and the heat-sensitive paper using the base paper, or to a heat-sensitive paper only. In this case, however, in the heat-sensitive paper the smoothness of the surface of which is improved by the calender treatment, with the improvement of recording density by the increase of the smoothness, the occurrence of sticking, piling, etc., of the heat-sensitive paper on a thermal head is increased, and hence the smoothness thereof is, in reality, controlled to a proper level to properly balance the recording density with the sticking or piling tendencies. Such conventional techniques cannot practically be applied for high-speed recording in point of recording density or recording stability to reduce the sticking and piling regardless of the selection of the smoothness level.
Sticking is a phenomenon in which a heat-sensitive color forming layer of the heat-sensitive recording paper sticks to a thermal head during recording to generate peeling sound and/or reduce the dot reproducibility, and piling is a phenomenon in which the heat melt of a heat-sensitive color forming layer is piled on a thermal head to reduce recording density and dot reproducibility. Both of these phenomena disturb stable recording in heat-sensitive recording systems.
Also, another demerit of the calender treatment for a heat-sensitive paper is that the heat-sensitive paper forms colored fog due to pressure, to thereby cause a high density in the background portions of the heat-sensitive recording paper. On the other hand, at present, there is a limit about a calender treatment onto a base paper owing to the formation of cockle, wrinkless, etc., caused by uneven basis weight of the heat-sensitive recording layer.
As described above, there remain limits on the smoothening of a heat-sensitive color forming layer and the increase of recording density that is possible by a calender treatment. Thus, a sufficiently satisfactory heat-sensitive recording paper for high-speed recording has not yet been obtained by the application of a calender treatment.